The invention relates to the field of magnetoresistive sensors (heads) and more particularly to magnetoresistive heads used in data storages systems and even more particularly to magnetic tunnel junction (MTJ) heads.
A typical prior art head and disk system 10 is illustrated in FIG. 1. In operation the head 20 is supported by the suspension 13 as it flies above the disk 16. The magnetic sensor, usually called a xe2x80x9chead,xe2x80x9d is composed of elements that perform the task of writing magnetic transitions (the write head 23) and reading the magnetic transitions (the read head 12). The electrical signals to and from the read and write heads 12, 23 travel along conductive paths (leads) 14 which are attached to or embedded in the suspension 13. Typically there are two electrical contact pads (not shown) each for the read and write heads 12, 23. Wires or leads 14 are connected to these pads and routed in the suspension 13 to the arm electronics (not shown). The disk 16 is attached to a spindle 18 that is driven by a spindle motor 24 to rotate the disk 16. The disk 16 comprises a substrate 26 on which a plurality of thin films 21 are deposited. The thin films 21 include ferromagnetic material in which the write head 23 records the magnetic transitions in which information is encoded. The read head 12 reads magnetic transitions as the disk rotates under the air-bearing surface of the head 20.
There are several types of read heads 12, which are called transducers and sensors interchangably, including those using spin valves and tunnel junctions. Heads using spin valves are called GMR heads. The basic structure of a spin valve sensor (not shown) includes thin films for an antiferromagnetic layer, a pinned layer and a free layer. The spin valve effect is a result of differential switching of two weakly coupled ferromagnetic layers. Magnetic tunnel junction (MTJ) devices. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The prior art MTJ device shown in FIG. 2 shows a section of a read head 12 with an MTJ thin film layer structure comprises a pinned ferromagnetic layer (pinned layer) 34 and a free ferromagnetic layer (free layer) 36 separated by a thin, electrically insulating, tunnel barrier layer 35. The tunnel barrier layer 35 is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. When an electric potential is applied between the pinned and free ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage.
The magnetization of the pinned layer 34 is fixed through exchange coupling with the antiferromagnetic (AFM) layer 33. The cap layer 37 separates the free layer 36 from the first lead 31A. The tunnel barrier layer 35 is a nonmagnetic, electrically insulating material such as aluminum (III) oxide (Al2O3), aluminum (III) nitride (AIN) and magnesium (II) oxide (MgO). The seed layer 32 is deposited prior to the layers shown above it and is used to establish the growth conditions and control the crystalline characteristics of layers following it. The first and second leads (31A, 31B) provide electrical connections for the flow of sensing current to a signal detector (not shown) that senses the change in resistance in the free layer 36 induced by the external magnetic field that is generated by the magnetic media.
Ferromagnetic materials most suitable for use as the pinned and free layers separated by the insulating tunnel barrier layer are materials with high spin polarization coefficients. Materials with high spin polarization coefficients near the tunneling junction are known to have higher magnetoresistance coefficients in MTJ sensors. A problem arises with some of the known materials that achieve the higher magnetoresistance coefficients is that they also may have high magnetostriction coefficients. When stressed the MTJ sensor layers with high magnetostriction coefficients can result in high uniaxial anisotropy fields in the pinned layer which can act to cancel part of the exchange field from the AFM layer resulting in reduced stability of the MTJ sensor especially at elevated operating temperatures. In addition, the stressed, high magnetostriction materials can result in high anisotropy fields in the free layer which reduces the sensitivity of the free layer to rotate in the presence of the external signal field. In order to eliminate undesirable magnetostriction, previous MTJ sensors have used ferromagnetic materials such as permalloy (Co90Fe10) which have very small magnetostriction coefficients, but which also have smaller magnetoresistance coefficients.
In U.S. Pat. No. 6,127,045 to Gill a magnetic tunnel junction (MTJ) device is described which has a high spin polarization ferromagnetic layer (Ni40Fe60) is placed near the tunnel barrier layer in both the pinned and free layers to enhance the magnetoresistive effect. The undesirable positive magnetostriction coefficient of the Ni40Fe60 layers is cancelled by placing a negative magnetostriction layer (Ni90Fe10) of the appropriate thickness adjacent to each Ni40Fe60 layer. The thicknesses of the positive and negative magnetostriction layers are chosen so that the net magnetostriction of the pinned layer and the free layer is approximately zero.
What is needed is a structure for an MTJ sensor which allows the use of materials for free layer that result in the highest magnetoresistive coefficients without degradation in sensitivity and thermal stability due to uncontrolled effects from magnetostrictive properties of these materials.
A tunnel junction sensor according to the invention replaces the prior art free layer with a free layer structure which allows a wider range of magnetoresistive materials to be used. The preferred free layer structure 40 of the invention includes a negative magnetostriction layer which allows use of magnetoresistive materials which otherwise have unacceptably high magnetostriction values. The materials and thicknesses of the layers result in a total magnetostriction near zero even though ferromagnetic material with high magnetostriction is included. This allows materials with high positive magnetoresistive constants such as Co50Fe50 and iron to be used in the free layer structure in contact with the barrier layer without the deleterious effects of high magnetostriction. In one embodiment, the invention uses bcc iron with a MgO barrier layer, since bcc iron yields particularly high magnetoresistive when used with a MgO barrier. In the preferred embodiment of the invention a layer of material with a negative magnetostriction constant such as nickel or an amorphous cobalt alloy is used to achieve a combined magnetostriction of near zero. The preferred embodiment also includes a softening layer of material such as selected compositions of nickel-iron which maintain the desired magnetic softness of the free layer structure and have a magnetostriction constant near zero.